Optimized heat exchange in a co2 de-sublimation process

ABSTRACT

The present invention is a process for removing carbon dioxide from a compressed gas stream including cooling the compressed gas in a first heat exchanger, introducing the cooled gas into a de-sublimating heat exchanger, thereby producing a first solid carbon dioxide stream and a first carbon dioxide poor gas stream, expanding the carbon dioxide poor gas stream, thereby producing a second solid carbon dioxide stream and a second carbon dioxide poor gas stream, combining the first solid carbon dioxide stream and the second solid carbon dioxide stream, thereby producing a combined solid carbon dioxide stream, and indirectly exchanging heat between the combined solid carbon dioxide stream and the compressed gas in the first heat exchanger.

BACKGROUND

Carbon dioxide emissions from fossil fuel combustion is a growing sourceof concern, and various technologies have been, and continue to bedeveloped, for capturing CO2 from flue gas and other gas streams. Majortechnologies include amine wash, physical adsorption technologies,cryogenic technologies (CO2 liquefaction). These technologies involvesignificant additional investment and operating costs for industrialplants. In the case of coal power plants for example, a resultingincrease of the cost of electricity in the range of 4 to 5 US cents/kWhis expected. One of the main challenges today is to reduce cost ofcarbon dioxide capture from flue gas through efficiency improvement andcapital reduction.

There are major drawbacks to all the existing systems. One possiblealternative to traditional capture solutions is called anti-sublimation.The basic concept is to separate CO2 from a flue gas by cooling the fluegas to turn the CO2 into solid (de-sublimation or cryo-condensation ofCO2). Indeed, at such low CO2 partial pressure (<5.11 atmosphere), theCO2 will be directly changed from gas phase to solid phase. There aretwo main schemes to implement such a process. The first isde-sublimation at atmospheric or very low pressure. For this scheme, asignificant external refrigeration loop is required to perform such acooling. This is known as indirect de-sublimation. The second isde-sublimation at higher pressure by expansion with solid formation.This is known as direct de-sublimation.

However, in any case, the efficiency of the process is strongly relatedto the ability to integrate the heat exchange. This is to say thatwithout heat exchangers to recover energy from the flue gas notably, theefficiency of the process would be drastically decreased. Hence, thereis a need in the industry for an optimized heat exchange in a carbondioxide de-sublimation process.

SUMMARY

The present invention is a process for removing carbon dioxide from acompressed gas stream including cooling the compressed gas in a firstheat exchanger, introducing the cooled gas into a de-sublimating heatexchanger, thereby producing a first solid carbon dioxide stream and afirst carbon dioxide poor gas stream, expanding the carbon dioxide poorgas stream, thereby producing a second solid carbon dioxide stream and asecond carbon dioxide poor gas stream, combining the first solid carbondioxide stream and the second solid carbon dioxide stream, therebyproducing a combined solid carbon dioxide stream, and indirectlyexchanging heat between the combined solid carbon dioxide stream and thecompressed gas in the first heat exchanger.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While theinvention is susceptible to various modifications and alternative forms,specific embodiments thereof have been shown by way of example in thedrawings and are herein described in detail. It should be understood,however, that the description herein of specific embodiments is notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

This invention relates to a specific process embodiment where the heatexchange is optimized in order to maximize efficiency in a realistic andcost-effective way.

The compressed flue gas stream 101 is first cooled down to abovefrosting point temperature in a multi-fluid heat exchanger 102 which maybe a brazed aluminium heat exchanger (such as is a typical cryogenicheat exchanger used in Air Separation Units).

After multi-fluid heat exchanger 102, the refrigerated flue gas 103 issent to a de-sublimating heat exchanger 104 where part of the CO2 isde-sublimated 113.

After the de-sublimating heat exchanger 104, the partially CO2 depletedflue gas 105 is sent to an expansion turbine 106 where it is expanded toalmost atmospheric pressure 107 and the remainder of the CO2 to becaptured is recovered as solids 109 in solids separator 108, (i.e.,achieving 90% capture of the incoming CO2 flow rate when streams 113 and109 are combined).

The solid CO2 streams 113 and 109 are mixed together 114 and pumped to ahigh pressure 116 in solids pressurizer 115. This pressure should behigh enough to not vaporize at ambient temperature. Then the highpressure CO2 stream 116 is sent to a melting heat exchanger 117 wheremost of the sensible heat plus the latent heat of fusion is recovered bythe condensing refrigerant 123. The liquefied CO2 118 is then sent tothe multi-fluid heat exchanger 102 to recover the sensible heat of theliquid, after which the warmed fluid 119 exits the system. The002-depleted cold gas 110 is sent to the de-sublimating heat exchanger104 and then to the multi-fluid heat exchanger 102 to recover all thecold. It is then pumped to the final pressure (not shown).

A small refrigeration loop is required in this configuration. Therefrigerant cycle is an inversed rankine cycle, the condensation happensin the melting heat exchanger 117, close to CO2 triple pointtemperature, and vaporization happens in the de-sublimating heatexchanger 104, below the outlet temperature of the de-sublimating heatexchanger. However, all other heat recovery involved will happen in themulti-fluid heat exchanger 102.

The following points are important in order to achieve a highefficiency. There needs to be partial de-sublimation of the flue gasprior to the turbine, thus allowing less temperature difference betweenthe inlet and the outlet of the turbine. This also allows the pressurerequired for the flue gas to be as low as approximately 6 bar absolute.There needs to be an inversed rankine cycle of the refrigerant withcondensation happening at CO2 melting temperature There needs to be heatrecovery of all fluids not involving solid CO2 in the multi-fluid heatexchanger (aluminium brazed in particular). The heat integration betweenflue gas lines, CO2 lines and refrigerant lines allows lowering theaverage temperature difference. Furthermore, a very low temperatureapproach (down to 2 C or below) would be achievable whereas typically 5C is a reasonable limit in other types of heat exchanger.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of the presentinvention.

What is claimed is:
 1. A process for removing carbon dioxide from acompressed gas stream comprising: a) cooling the compressed gas (101) ina first heat exchanger (102), b) introducing the cooled gas (103) into ade-sublimating heat exchanger (104), thereby producing a first solidcarbon dioxide stream (113) and a first carbon dioxide poor gas stream(105), c) expanding the carbon dioxide poor gas stream, therebyproducing a second solid carbon dioxide stream (109) and a second carbondioxide poor gas stream (110), d) combining the first solid carbondioxide stream and the second solid carbon dioxide stream, therebyproducing a combined solid carbon dioxide stream (114), and e)indirectly exchanging heat between the combined solid carbon dioxidestream and the compressed gas in the first heat exchanger.
 2. Theprocess of claim 1, wherein the compressed gas is flue gas.
 3. Theprocess of claim 1, wherein the compressed gas is cooled to atemperature higher than the carbon dioxide de-sublimation temperature.4. The process of claim 1, wherein the second carbon dioxide poor gasstream (110) is introduced into the de-sublimating heat exchanger (104)to indirectly exchange heat with the cooled gas (103).
 5. The process ofclaim 1, wherein after the second carbon dioxide poor gas stream exitsthe de-sublimating heat exchanger, it (111) is introduced into the firstheat exchanger (102) to indirectly exchange heat with the compressed gas(101).
 6. The process of claim 1, further comprising introducing thecombined solid carbon dioxide stream (114) into a second heat exchanger(117), after step d) and before step e), wherein the combined solidcarbon dioxide stream indirectly exchanges heat with an externalrefrigeration loop (123).
 7. The process of claim 6, wherein theexternal refrigeration loop comprises: vaporizing a compressedrefrigeration stream (125/127) in the de-sublimating heat exchanger(104), thereby producing a warmed refrigeration stream (129/130/120),compressing the warmed refrigeration fluid (120), thereby producing acompressed refrigeration stream (122), and condensing the compressedrefrigeration stream in a second heat exchanger (117) by indirectlytransferring heat with the combined solid carbon dioxide stream(114/116), thereby producing a cooled refrigeration stream (124).
 8. Theprocess of claim 6, wherein the compressed refrigeration stream (122) isfurther warmed (123) in the first heat exchanger (102) before indirectlytransferring heat with the combined solid carbon dioxide stream.
 9. Theprocess of claim 6, wherein the cooled refrigeration stream (124) isfurther cooled (125) in the first heat exchanger (102), before beingcompressed.